1. Field of the Invention
The present invention relates to solid-state imaging devices, manufacturing methods and designing methods thereof, and electronic devices, particularly to solid-state imaging devices that include R (red), G (green), and B (blue) color filters, manufacturing methods and designing methods thereof, and electronic devices.
2. Description of the Related Art
Electronic devices such as digital video electronic devices and digital still electronic devices include solid-state imaging devices, for example, such as CCD (Charge Coupled Device) image sensors, and CMOS (Complementary Metal-Oxide-Silicon Transistor) image sensors.
Such solid-state imaging devices include a plurality of pixels that is disposed in a matrix along the horizontal and vertical directions to form a light-receiving surface on a semiconductor substrate. On the light-receiving surface, a sensor, for example, such as a photodiode, is provided as a photoelectric converting section for each pixel. A light condensing structure that condenses the light of a subject image onto the sensor of each pixel is formed on the light-receiving surface. Once received, the light of a subject image is subjected to photoelectric conversion, and signal charges are generated to produce pixel signals.
In solid-state imaging devices that form color images, a color filter of red (R), green (G), or blue (B) is formed corresponding to each pixel.
FIG. 20A is a cross sectional view of a pixel of a solid-state imaging device of related art, representing the older generation of solid-state imaging device with the cell size of about 3 μm.
A photodiode 111 is formed on a semiconductor substrate 110 for each pixel, and a gate insulating film and gate electrodes (not illustrated) are formed on regions adjacent to the photodiode 111. The gate electrodes are controlled to perform processes that include transfer of the accumulated signal charges in the photodiode 111.
For example, a bottom-layer first insulating film 120a of, for example, silicon oxide, is formed over the whole surface, covering the components formed on the semiconductor substrate 110, including the photodiode 111 and the gate electrodes, so as to planarize irregularities created by the gate electrodes, etc.
For example, a second insulating film 120b of, for example, silicon nitride, and a third insulating film 120c of, for example, resin are formed on the first insulating film 120a. A fourth insulating film 121 of, for example, silicon oxynitride is formed thereon.
For example, a fifth insulating film 122 of, for example, silicon nitride is formed on the fourth insulating film 121.
A color filter 123 that transmits the light of wavelength, for example, in the red (R), green (G), or blue (B) region is formed for each pixel on the fifth insulating film 122. An on-chip lens 124 is formed on the color filter 123.
In the pixel of each color, the photodiode sensor provided for the pixel receives the light of wavelength corresponding to each color, and pixel signals for forming color images are obtained.
As the movement toward miniaturization of semiconductor integrated circuits continues, the condensing structure of the solid-state imaging device has become more complex than ever before. The finer device structure involving process variation complicates the light paths of the light passing through the condensing structure. This presents the problem of color nonuniformity during imaging.
Color nonuniformity is a phenomenon that occurs as a result of disrupted color balance in the incident light, caused by the wavelength-dependent intensity variation of the incident light on the sensor due to different thicknesses of the films forming the condensing structure.
An example of such color nonuniformity is color frame nonuniformity, which occurs when the color balance of the transmitted light is varied and disrupted at the central portion and the peripheral portion of the light-receiving surface by thickness variation, or intra-chip thickness variation as it is called, in which the extent of thickness changes in the film forming the condensing structure varies for the central portion and the peripheral portion of the light-receiving surface of the solid-state imaging device.
Color nonuniformity adds color to the field angle of image data, and thus reduces the yield of imaging device. Color nonuniformity becomes problematic in the generation of device with the cell size smaller than about 3 μm.
In the older generations with the cell size no smaller than about 3 μm, the on-chip lens and other lenses in the device bend the light, creating various angular components in the incident light on the sensor.
FIGS. 20B and 20C represent thickness dependence of the sensor intensity of the sensor of the foregoing configuration. The horizontal axis represents the thickness of a high-refractive-index film that has the interface that interferes with the reflected incident light at the silicon semiconductor substrate interface.
In the sensor of the foregoing configuration, as represented in FIG. 20B, the peaks and troughs of the interference light cancel out, and produce the thickness dependence as represented in FIG. 20C, i.e., sensor intensity with small thickness dependence.
In devices with a large cell pitch, the sensor intensity is constant for each wavelength of RGB, even when the thickness of the high-refractive-index film is varied by process variation, and color nonuniformity does not easily occur.
FIG. 21A is a cross sectional view of a pixel of a solid-state imaging device of related art with a cell size smaller than about 3 μm. This configuration is the same as that illustrated in FIG. 20A, except that the cell size is simply smaller.
In the pixel of such a configuration, the layer thickness remains the same despite the smaller cell pitch, and therefore, as illustrated in FIG. 21A, light is not bent through the lens in the device, and enters the sensor in the form of almost parallel rays with fewer angular components.
The light incident in this manner has only a single interference light component, and accordingly there is no canceling out of different light components.
FIG. 21B represents thickness dependence of the sensor intensity of the sensor of the foregoing configuration.
Because the thickness dependence of the single-component sensor intensity does not cancel out and remains, the sensor intensity varies with variation in the thickness of the high-refractive-index film, as represented in FIG. 21B.
FIG. 22 is a graph representing cell pitch plotted against the difference in sensitivity ratio (R/G) between a pixel at the center of a field angle and a pixel at an edge of the field angle. The graph has been normalized, and the vertical axis represents the difference in sensitivity ratio δ (relative value).
A similar graph can be obtained by normalizing the difference in sensitivity ratio (B/G) between a pixel at the center of the field angle and a pixel at an edge of the field angle.
Differences in sensitivity ratio between the center and edge of the field angle occur with cell pitches of 3 μm and smaller, showing that the cell pitch of 3 μm is the critical boundary. These differences are the cause of color nonuniformity.
In order to overcome the problem of color nonuniformity, JP-A-2007-242697 (Patent Document 1) proposes a structure that reduces color nonuniformity by suppressing interference using an antireflection film formed over and underneath a film of a high refractive index.
JP-A-6-292206 (Patent Document 2) proposes a device that includes a reflection preventing structure to reduce reflection of light.
JP-A-2005-142510 (Patent Document 3) proposes a device in which the thickness of an antireflection film directly above a light receiving section is varied for each different wavelength to suppress the intensity of reflected light, and to thereby improve light receiving efficiency.
The method of Patent Document 1 is intended to suppress color nonuniformity by reducing reflection at the high-refractive-index film that produces the color nonuniformity-causing interference light. For this purpose, the antireflection film of a constant thickness is formed over and beneath the high-refractive-index film. The constant thickness of the antireflection film is the average of the thickness that reduces red (R) reflected light near the wavelength 620 nm, and the thickness that reduces green (G) reflected light near the wavelength 550 nm.
However, color nonuniformity cannot be suppressed effectively with this method, because the method does not take into account the wavelength of blue (B) light near 440 nm, and because the interference light that causes color nonuniformity is sensitive to the thicknesses of the antireflection film and the high-refractive-index film.
This is explained below with reference to the structure illustrated in FIG. 21A. The structure illustrated in FIG. 21A represents a common structure of solid-state imaging devices such as CCD image sensors and CMOS image sensors.
Components such as the gate insulating film and gate electrodes (not illustrated) are formed on the semiconductor substrate 110 in regions adjacent to the photodiode 111 formed on the semiconductor substrate 110 for each pixel. The gate electrodes are controlled to perform processes that include transfer of the accumulated signal charges in the photodiode 111.
For example, a bottom-layer first insulating film 120a of, for example, silicon oxide, is formed over the whole surface, covering the components formed on the semiconductor substrate 110, including the photodiode 111 and the gate electrodes, so as to planarize irregularities created by the gate electrodes, etc.
For example, a second insulating film 120b of, for example, silicon nitride, and a third insulating film 120c of, for example, resin are formed on the first insulating film 120a. A fourth insulating film 121 of, for example, silicon oxynitride is formed thereon.
For example, a fifth insulating film 122 of, for example, silicon nitride is formed on the fourth insulating film 121.
Color nonuniformity occurs, for example, as a result of variation in optical interference intensity due to the reflected incident light at the surface of the semiconductor substrate, and the reflected light at the interface between the fourth insulating film 121 and the fifth insulating film 122.